Molecular engineering of non-centrosymmetric Langmuir-Blodgett films for NLO applications: Films with interlocking Lego-type structures

The cationic dye, E-N-alkyl-4-[2-(4-alkyloxy-naphthyl) ethenyl] quinolinium bromide has a wide-bodied chromophore with alkyl groups at opposite ends, i.e. CmH2m+1- D-pi-A(+)-CnH2n+1 X- where D and A are electron-donor and ele ctron-acceptor groups respectively, pi is a conjugated bridge and X- is the bromide counterion. It forms non-centrosymmetric Langmuir-Blodgett (LB) st ructures in which the packing is de pendent upon the combination and compat ibility of the alkyl groups. Films of two isomeric analogues where m + n = 34, have a layer thickness of 4.3 nm when the groups are significantly diff erent (m = 12 and n = 22) which decreases to 2.5nm when they are similar (m = 18 and n = 16). The latter adopts an interdigitating layer arrangement a t both the upper and lower interfaces and it is the first example of a Lcgo -type structure with non-centrosymmetric alignment of the optically nonline ar chromophores. This is evidenced by the second-harmonic intensity, which increases quadratically with the number of layers, i.e. as I-(N)(2w) = (I(1 )N2)-N-2 omega.